Wind turbine providing grid support

ABSTRACT

A variable speed wind turbine is arranged to provide additional electrical power to counteract non-periodic disturbances in an electrical grid. A controller monitors events indicating a need to increase the electrical output power from the wind turbine to the electrical grid. The controller is arranged to control the wind turbine as follows: after an indicating event has been detected, the wind turbine enters an overproduction period in which the electrical output power is increased, wherein the additional electrical output power is taken from kinetic energy stored in the rotor and without changing the operation of the wind turbine to a more efficient working point. When the rotational speed of the rotor reaches a minimum value, the wind turbine enters a recovery period to re-accelerate the rotor to the nominal rotational speed while further contributing to the stability of the electrical grid by outputting at least a predetermined minimum electrical power.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 13/381,630, filed Dec. 29, 2011. The aforementioned relatedpatent application is herein incorporated by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to a variable speed wind turbine for connecting toan electric grid and being arranged to generate additional electricaloutput power on the occurrence of grid disturbances. More particularly,the invention is directed to a wind turbine which is able to furthersupport the stability of the electrical grid after the output of theadditional electrical power has finished.

BACKGROUND OF THE INVENTION

Unbalances between the electrical power fed into an electrical grid andthe electrical power withdrawn from it lead to fluctuations of the gridfrequency. If the electrical power generation drops below the powerconsumption from an electrical grid, e.g. due to a power plant failureor disconnection, the grid frequency drops. Conversely, if the powerconsumption falls below the amount of electrical power generation, thegrid frequency increases. In order to compensate for such frequencyfluctuations, there are power generating stations which are arranged tocontinuously vary their active electrical power output until theunbalance has been eliminated. This electrical output power variation iscalled “primary power control”. Grid operators specify the primary powercontrol requirements in so-called Grid Codes as, for example, the GridCode 2006 by E.ON Netz GmbH, English version, published by E.ON NetzGmbH, downloadable athttp://www.eon-netz.com/pages/ene_de/Veroeffentlichungen/Netzanschluss/Netzanschlussregeln/ENENARHS2006eng.pdf.

Currently, wind turbines generally do not contribute to primary powercontrol, mainly because the power source “wind” is not controllable.However, with the increasing proportion of wind energy plants in theoverall electrical power production, a contribution of wind turbines toprimary power control is desired.

Also, mechanical rotating parts of the energy conversion system ofmodern variable speed wind turbines are not electrically coupled to theelectricity network, thus the wind turbine is mechanically decoupledfrom the grid, differently to conventional fixed speed synchronousgenerators. In this way, modern wind turbines do not have an inherentcontribution to the grid stability when a grid event is experienced,such as sudden imbalances between total generation and consumption inthe network (due to generator trip or load trip), differently fromconventional fixed speed synchronous generators. Such wind turbines arethus not contributing with the grid rotating inertia. With theincreasing proportion of wind energy plants in the overall electricalpower production, the number of fixed speed synchronous generators isdecreasing, thus loosing the inherent capability of generation mix tosupport grid stability when a grid event is experienced such as suddenimbalances between total generation and grid consumption. The total gridinertia is decreased, deteriorating the grid frequency stability. Acontribution of wind turbines with fast controlled active powermodulation for grid stability is desired.

It is known, for example from DE 100 22 974 A1, that wind turbines canreact to grid frequency increases (i.e. less power is consumed from theelectrical grid than is fed into it) by decreasing their output power.It is, however, hard to respond to frequency decreases (i.e. more poweris consumed from the electrical grid than is fed into it) because thatmeans increasing the active electrical power production without havingmore wind energy available. Two different approaches are known toaddress this issue:

Firstly, two papers by Harald Weber et al. from Rostock University(“Netzregelverhalten von Windkraftanlagen”, published at the conference6^(th) GMA/ETG-Fachtagung “Sichere und zuverlässige Systemführung vonKraftwerk und Netz im Zeichen der Deregulierung”, held from 21^(st) to22^(nd) May 2003 in Munich, downloadable atwww.e-technik.uni-rostock.de/ee/download/publications_EEV/uni_hro_publ35_WKA_(—)2003.pdf, “Primärregelung mit Windkraftanlagen”, published atthe ETG-Workshop “Neue dezentrale Versorgungsstrukturen”, held from19^(th) to 20^(th) February 2003 in Frankfurt/Main, downloadable atwww.e-technik.uni-rostock.de/ee/download/publications_EEV/uni_hro_publ33_etg_frankfurt_(—)2003.pdf,both documents are hereinafter referred to as the “Rostock papers”)recommend the operation of a wind turbine at a suboptimal working point(e.g. at a higher than optimal rotational rotor speed, at a given windspeed) in order to have power reserves available which can beadditionally output in the case of a frequency drop (e.g. by thenlowering the rotational rotor speed to the optimal speed, at a givenwind speed). By this, the additional electrical output power can be fedinto the grid over an indefinite time.

According to the second approach, which is for example outlined in WO2005/025026 A1, the kinetic energy stored in a wind turbine's rotor isidentified as a power reserve that can be transformed into electricalpower and additionally injected into the grid, however, only over ashort time period. By using kinetic rotor energy, it is also possible tocompensate periodic frequency oscillations, by periodicallyde-accelerating and acceleration the rotor, in synchronisation with thefrequency oscillation.

A similar concept for short-time power input at the cost of kineticrotor energy is provided by the article “Temporary Primary FrequencyControl Support by Variable Speed Wind Turbines—Potential andApplications” by Ullah et al. (published by IEEE in “IEEE Transactionson Power Systems, Vol. 23, No. 2” in May 2008, pages 601 to 612,downloadable at ieeexplore.ieee.org/iel5/59/4494587/04480153.pdf;hereinafter referred to as “ULLAH”).

These proposals for using kinetic energy from the rotor to temporarilyoutput additional electric power are, however, not yet matured as saiddocuments are not concerned with wind turbine controlling after anon-periodic additional electrical power output has ended. The presentinvention provides a refined approach for a fast active power variationfor grid stability and primary power control contribution by windturbines.

SUMMARY OF THE INVENTION

According to a first aspect, the invention provides a variable speedwind turbine for connection to an electrical grid. The wind turbine isarranged to provide additional electrical power to counteractnon-periodic disturbances in the electrical grid. It comprises a rotorwith blades coupled to an electrical generator and a controller whichmonitors events indicating a need to increase the electrical outputpower from the wind turbine to the electrical grid to support thestability of the electrical grid. The controller is arranged to controlthe wind turbine to perform grid-stability supporting activity, in anon-periodic manner, as follows: After an indicating event has beendetected, the wind turbine enters an overproduction period in which theelectrical output power is increased beyond the normal electricaloperating power, wherein the additional electrical output power is takenfrom kinetic energy stored in the rotor and without changing theoperation of the wind turbine, when working in at least a partial-loadmode, to a more efficient working point. At the latest, when therotational speed of the rotor reaches a minimum value, the wind turbineenters a recovery period to re-accelerate the rotor to the nominalrotational speed while further contributing to the stability of theelectrical grid by outputting at least a predetermined minimumelectrical power to the electrical grid.

According to a second aspect, the invention provides a controllerarranged to control a variable speed wind turbine accordingly.

According to a third aspect, the invention provides a correspondingmethod of controlling a wind turbine for providing additional electricalpower.

Further aspects are set forth in the dependent claims, the followingdescription and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are explained by way of example with respect to theaccompanying drawings, in which:

FIG. 1 schematically shows an assembly of a variable speed wind turbine;

FIG. 2 is a simplified diagram depicting a control flow of the windturbine;

FIG. 3 exemplarily illustrates a wind turbine's production curve;

FIG. 4 is a diagram showing the correlation between rotational speed ofthe rotor and the electrical output power according to a firstembodiment;

FIG. 5 illustrates the developing of the electrical output power, theavailable mechanical blade power and the rotor speed according to thefirst embodiment;

FIG. 5 a is a schematic illustration of the first embodiment accordingto FIG. 5, wherein the electrical output power is modulated during theoverproduction and recovery period;

FIG. 6 depicts the correlation between rotational speed of the rotor andthe electrical output power according to a second embodiment;

FIG. 7 is a diagram showing the characteristics of the electrical outputpower, the available mechanical blade power and the rotor speedaccording to the second embodiment;

FIG. 8 shows the electrical and mechanical power curves as well as therotor speed according to a third embodiment;

FIG. 9 illustrates the correlation between rotational speed of the rotorand the electrical output power in a situation in which the availablewind power is higher than the nominal electrical power of the windturbine (full-load operation);

FIG. 10 schematically shows the mechanical and electrical components ofa wind turbine with a synchronous generator and a full-scale converter;

FIG. 11 schematically shows the mechanical and electrical components ofa wind turbine with a Doubly-Fed Induction Generator (DFIG);

FIG. 12 exemplarily shows the duration of the recovery period fordifferent levels of electrical output power in dependency on the windspeed;

FIG. 13 illustrates the method of grid-stability support according toone aspect of the present invention.

NOMENCLATURE

The following terms and abbreviations are used throughout thisspecification:

-   -   P_(nom): is the nominal (rated) electrical operating power of        the wind turbine according to the invention (when referring to        “power”, we mean the physical unit with the dimension        energy/time or work/time);    -   P_(e): is the actual electrical operating (output) power at a        certain point in time;    -   P_(e0): stands for the (normal) wind turbine's electrical        operating power at the moment of entering the power control mode        according to the present invention;    -   ΔP_(op): is the overproduction power which is output in addition        to the normal electrical operating power of the wind turbine        during the overproduction period;    -   P_(emin): designates the predetermined minimum electrical output        power during the recovery period;    -   P_(acc): stands for the power used for re-accelerating the rotor        during the recovery period according to the present invention;    -   P_(w): denotes the available wind power at a certain point in        time;    -   P_(m): stands for the wind turbine's mechanical rotor power at a        certain point in time;    -   ω₀: designates the rotational speed of the wind turbine's rotor        at the moment of entering the power control mode according to        the present invention;    -   ω_(min) is the minimum rotational speed of the wind turbine's        rotor at which the wind turbine enters the recovery period.

GENERAL DESCRIPTION

Before turning to the detailed description of the embodiments, a fewgeneral items will first be discussed.

The main components of the wind turbine according to the invention are arotor with blades which is coupled to an electrical generator and acontroller which is arranged to control the wind turbine to performgrid-stability supporting activity. The wind turbine is a variable speedwind turbine, i.e. the rotor speed can be varied during ongoingoperation in order to permit the efficient exploitation of the availablewind power while minimising wind turbine load and abrasion. It isfurther arranged for connection to an electrical grid which is, forexample, a three-phased public power supply network or, if the windturbine belongs to a wind park, the wind park's internal electricalvoltage network (which itself may, in general, again be connected to apublic power supply network). To enable the wind turbine to be operatedwith variable rotor speeds, it is mechanically decoupled from the fixedgrid frequency, e.g. by an electric converter or a variable-speedgear-box between rotor and generator.

During operation in partial-load mode (i.e. the available wind power isbelow the wind turbine's rated wind power), the wind turbine generallyoperates at an optimal working point. Depending on the wind speed anddirection, the rotor speed as well as pitch and yaw angles (assumingthat the wind turbine is equipped with blade pitch and yaw controlsystems) are continuously adjusted so that maximum aerodynamicefficiency is maintained (e.g. the wind turbine's tip-speed ratio ismaintained at optimal value) and maximum electrical power is capturedfrom the wind conditions (this policy is usually called “maximum powertracking”). If the available wind power reaches or exceeds the windturbine's rated wind power, rotational rotor speed and the electricaloperating power are limited to their nominal values by, e.g.,controlling the blades' angles of attack using a blade pitch controlsystem. The wind turbine is thereby protected against overload while thegenerated electrical power is kept constant at its rated value(P_(nom)=P_(e)). Hence, only in this so-called full-load mode, theavailable wind power is not exploited as well as possible, but theelectrical output power is limited by turning the blades out of the windto a certain extent.

The “Rostock papers”, however, propose a wind turbine operationdeviating from this. In order to have a power reserve available forprimary power control, a wind turbine shall generally be operated in anon-optimal manner. This could be realised by driving the rotor at ahigher or lower rotational velocity or by turning the blades to asuboptimal pitch angle (cf. “Netzregelverhalten von Windkraftanlagen”,p. 6). Such a systematic operation off the optimal workingcharacteristic curve, however, causes significant waste of availablewind energy. From an economic point of view, this is problematicbecause—unlike power plants based on controllable energy sources likewater or gas plants where the unspent—and thus saved—amount of theenergy carrier can be converted to electrical power at latertimes—non-exploited wind energy is simply lost. Thus, the approachsuggested by the “Rostock papers” is not followed here, but the windturbine according to the invention operates on its normal torque-speedcharacteristic curve, at least when working in partial-load mode.

As outlined by ULLAH and WO 2005/025026 A1, it is possible to provideadditional output power—even if the wind turbine is operated at itsoptimal working point—by extracting kinetic energy from the rotatingrotor. According to ULLAH, it is for example possible to output anadditional level of electrical power—which is appropriate forcontributing to grid frequency maintenance—for about 10 seconds (incontrast to classical primary power control where the additionalelectrical power is generated from available power reserve and can,thus, be generally maintained over a long time). This transient andoccasional power increase is also called “grid inertia response”. Theextraction of kinetic energy will lead to a deceleration of the rotorand thus generally causes a deviation from the optimal operating pointfor the time of the frequency stability contribution (ULLAH, p. 608, r.col.). After the period of additional electrical power output (which iscalled “over-production period” hereinafter) the electrical output poweris rapidly decreased and power may even be consumed from the electricalgrid so that the rotor can be accelerated again (p. 609, 1. col.).However, this rapid power decrease causes a second grid frequency drop(cf. FIGS. 17 (a), 18 and 19) so that—after the initial short-terminertia response—subsequent frequency stabilization is not provided.

The present invention focuses on wind turbine control in the phase afterthe overproduction period has been ended and until the rotor has beenre-accelerated to its normal speed (this period is called “recoveryperiod” hereinafter). It provides a wind turbine arranged forcontributing to primary power control in a way that the stability of theelectrical grid is further supported during the recovery period.

WO 2005/025026 A1 pursues the same approach as described by ULLAH andadditionally mentions the utilisation of the rotor's kinetic energy fordampening periodic grid frequency oscillations (so-called “Inter AreaOscillations”, WO 2005/025026 A1, p. 14). In a given example, the gridfrequency oscillates with a characteristic frequency of 0.22 Hz (equalto a cycle duration of 4.5 s) which means that for dampening thisoscillation the wind turbine injects additional power for 2.25 s andreduces power output for another 2.25 s in an alternating manner (p. 14and FIG. 3). In this scenario, the significant electrical output powerreduction following the increase beyond the normal operating power isintended (because the grid frequency is above nominal for the respectivehalf of the cycle) so that there is no need to counteract an ongoingfrequency valley.

In contrast to this, the present invention is conceived to counteractnon-periodic disturbances in the electrical grid. In particular, if thegrid frequency drops for longer periods due to longer-term increasedpower consumption from the electrical grid or shutdown/failure ofanother power plant, the wind turbine operated according to the presentinvention is capable of providing additional electrical power in atransient manner and still contributing to the grid stability after theadditional electrical power generation has ended.

In particular, the wind turbine according to the present invention isarranged to perform a two-phased ancillary control mode, the first phasebeing an overproduction period in which the electrical power suppliedfrom the wind turbine to the electrical grid is increased beyond thenormal electrical operating power, and the second phase being a recoveryperiod in which the wind turbine further contributes to the stability ofthe electrical grid. This further contribution is achieved by outputtingan amount of electrical power which is at or above a predetermined lowerthreshold. In this way, a (second) significant decrease of the gridfrequency, as occurs in ULLAH, can be diminished.

The wind turbine according to the invention comprises a controller whichis responsible for controlling the wind turbine's entry into thetwo-phased control mode. For this purpose, the controller (continuouslyor periodically) monitors events which indicate that the output ofelectrical power to the grid shall be increased beyond the normal, i.e.present electrical operating power P_(e). Such an indicating event can,for example, be a control signal generated by a wind turbine-externalentity such as the grid operator or a wind park controller.Alternatively, it can be an alarm triggered by a wind turbine-internalmeasurement of operating parameters, such as the grid frequency orvoltage angle change and, in particular, the determination of anoperating parameter deviation from its normal value to a predefinedextent. The indicating event can include not only the indication that anelectrical output power increase is needed, but also further informationabout the nature of the required power increase. For example, it couldcontain information about the magnitude of the power increase (as arelative or absolute value), the magnitude of the respective frequencydrop, the desired duration or variation of the electrical output powerincrease or other/additional management or meta data. The controller canthen be arranged to evaluate and process this information and toinitiate corresponding control activities.

In particular, after detection of an indicating event, the controllerinitiates a grid-stability supporting activity, starting with theoverproduction period. During this time the stability of the grid issupported by outputting electrical overproduction power (ΔP_(op)) on topof the normal electrical operating power which was being produced at thetime of the start of the overproduction period (P_(e0)). Hence, theelectrical output power during the overproduction period P_(e) can bespecified by P_(e)=P_(e0)+ΔP_(op) (assuming that the wind conditions donot change during the overproduction period, and, thus, the hypotheticalworking point at which the wind turbine would have been operated if ithad not entered the overproduction period had been maintained, i.e.P_(e)=P_(e0) during this time interval). The additional overproductionpower ΔP_(op) can be a predetermined and fixed value (the same value forany overproduction period and unchanged during a complete overproductionperiod), or variable case by case, but still constant over eachoverproduction period, or it can be varied during an overproductioncycle, for example in response to further events monitored by thecontroller or other management data (which could have been, for example,provided together with the indicating event), so that by modulation ofthe electrical output power the wind turbine is able to contribute tothe stability of the electrical grid depending on the nature andcharacteristics of the grid instability.

The overproduction power ΔP_(op) is, at least when the wind turbineoperates in partial-load mode, not generated by changing the windturbine operation to a more favourable working point, but is extractedfrom the kinetic energy stored in the wind turbine's rotating masses,i.e. its rotor. Accordingly, the rotor speed decrease during theoverproduction period is dependent on the amount of energy extracted.

The considerations laid out herein are based on the assumption that theavailable wind power (P_(w)) stays constant during the two-phasedancillary control mode. On that assumption, the decrease of rotor speedduring the overproduction causes the wind turbine to deviate from theoptimal working point, accepting a (relatively slight) loss ofefficiency (which means that, in fact, the wind turbine operation isjust converse to the “Rostock papers”: normal operation at a workingpoint with maximum efficiency, a (negative) deviation from the normalworking point only occurring while performing grid-stability supportactivity during the two-phased ancillary control mode). If the windturbine is equipped with a blade pitch control system, the efficiencydecline can partly be compensated by adjusting the pitch angle of therotor blades while the rotor speed changes.

The rotor speed, however, shall not decrease below a certain minimumvalue. This threshold can be related to construction parameters of thewind turbine, e.g. the operating range of the wind turbine's generatoror converter, or to efficiency considerations, in particular, to theminimum electrical power that should be output to the electrical gridduring subsequent operation. At the latest, when the rotational rotorspeed reaches this minimum value, the controller terminates theoverproduction period and initiates the recovery period.

The recovery period is characterised by two tasks which are basicallycontrary to each other. On the one hand, the electrical grid may stillnot be stable (i.e. there is still more power consumption from the gridthan is being fed into it) so that the need for a certain electricaloutput power is still present. On the other hand, due to the decreasedrotor speed, the wind turbine is operating at a suboptimal working pointand can therefore not generate as much electrical power as prior to theoverproduction period. Hence, there is a conflict between themedium-term electrical grid stability and the re-acceleration of therotor for efficient long-term power production.

According to the present invention, in this second phase of theancillary control mode, the wind turbine outputs at least apredetermined minimum of electrical power (P_(emin)) to the electricalgrid. That means that the electrical output power P_(e) does not dropbelow this predetermined threshold P_(emin) over the complete durationof the recovery period (ergo, at any point in time during the recoveryperiod the electrical output power is at or above the predeterminedminimum value). In that way, the wind turbine continues to contribute tothe stability of the electrical grid. At the same time, the rotor isre-accelerated again by using remaining mechanical power of the rotor(P_(acc)). That means that, in general, not the complete mechanicalrotor power (P_(m)) is converted to electrical power (P_(e)) during therecovery period. Depending on the apportionment of the availablemechanical rotor power (P_(m)) into the part used for generatingelectrical output power (P_(e)) and the part used for rotorre-acceleration (P_(acc)), the re-acceleration may take a significantamount of time. It is, for example, possible that—for a certain periodof time—the complete supply of mechanical rotor power (P_(m)) isconverted into electrical output power (P_(e)) and the rotor is notaccelerated at all so that the recovery period may additionally beprolonged.

The overproduction and recovery periods constitute a non-periodiccontrol mode. As the focus is here on grid stability (opposed to quickrotor re-acceleration), the recovery period will generally be longerthan the overproduction period. Furthermore, after the recovery periodhas ended, there will generally be a phase of normal wind turbinecontrol and operation for an indefinite time. Only when the controllerdetects another indicating event (which can, from the perspective of thewind turbine, occur at any arbitrary point in time) the controllerinitiates another overproduction phase. Thus, there is no continuousprocess like alternating overproduction and recovery periods orre-entering an overproduction phase after a predetermined intervalfollowing the previous recovery period.

The minimum value of electrical output power (P_(emin)) during therecovery period could theoretically be determined as an absolute powervalue. However, as wind turbines operate under varying wind conditionsand, accordingly, the wind turbine according to the invention alsoperforms the grid-stability supporting activity under varying windconditions, this might not be useful in practice. Therefore, in someoptional further configurations of the invention, P_(emin) is defined asa fixed percentage of the electrical operating power which was beingsupplied to the electrical grid at the moment the turbine entered theoverproduction period (P_(e0)). In other variants, other referencevalues are chosen, for example an average value of the operating powerwithin a certain time window before the initiation of the overproductionperiod, the wind turbine's nominal electrical operating power (P_(nom))or the normal electrical power corresponding to the currently availablewind power (in the recovery period).

In some further configuration options, the minimum electrical outputpower (P_(emin)) during the recovery period amounts to 80% of theelectrical operating power which was supplied to the electrical gridprior to the overproduction period (P_(e0)). In other embodiments, thethreshold is set to 85% or 90% of P_(e0). It is possible to choose aneven higher value, for example 95% of P_(e0). The electrical grid isthen supported at a higher level during the recovery period. However,the remaining power available for rotor re-acceleration is thencorrespondingly smaller so that the recovery is prolonged and the windturbine operates at a suboptimal working point for a longer time.

In some further configurations, the controller is arranged not only toensure that the electrical output power does not drop below thepredetermined minimum during the recovery period, but also to controlthe power used for re-acceleration of the rotor (P_(acc)). As a result,the controller can make sure that the rotor acceleration does not fallbelow a certain minimum value. Furthermore, it is thereby possible to(continuously or periodically) estimate the remaining duration of therecovery period.

In order to be able to control the rotor acceleration power (P_(acc)),the wind turbine comprises sensor equipment for measuring the rotationalspeed of the rotor and the wind speed, the moment at the blade rootsand/or the torque at the rotor shaft in some optional configurations ofthe present invention. The rotor acceleration can then be controlled bythe controller by (continuously or periodically) measuring theseparameters and by calculating the available mechanical rotor power(P_(m)) from at least these measured parameters. Thus, the controlleralways “knows” the available mechanical power and is able to segment itinto the two parts during the recovery period, namely the first partwhich is used for conversion to electrical output power and whichensures that the output power does not fall below the given lowerthreshold, and the second part which is used for re-acceleration of therotor to its nominal speed. As a result, more specific prognosesconcerning the remaining duration of the recovery period are possible.

The overproduction period does not need to last until the predeterminedminimum rotational rotor speed (ω_(min)) has actually been reached. Forexample, when the electrical grid regains its stability fast, therecovery period can be initiated sooner as there is no need forinjecting increased electrical power to the electrical grid anymore.Also, there may exist a predetermined time limit for the duration of theoverproduction period in order to protect the wind turbine from a tooextended increased electrical power production (e.g. to preventoverheating of power conversion components). Thus, when such a timelimit for increased electrical power production is reached, the recoveryperiod may also be initiated prior to reaching the minimum rotationalrotor speed (ω_(min)). Another (additional) condition for entering therecovery period could be an upper limit of the amount of additionalenergy provided to the electrical grid during the overproduction period.Finally, the wind turbine may be arranged to abort the increasedelectrical power production and to enter the recovery period in responseto control signals or external events, e.g. stipulations received fromthe grid operator or a superordinate control entity.

Furthermore, in some variants, a time period is set in which the rotorspeed regains its nominal speed (which marks the end of the recoveryperiod). The controller can then vary both, rotor acceleration andelectrical output power (P_(acc) and P_(e)), so that the electricaloutput power may not only be modulated during the overproduction period(as mentioned above), but also during the recovery period in order toenable the wind turbine to contribute to the grid stability in alignmentwith the characteristics of the instability (for example, in response tocontrol signals provided by grid measurements or the electrical gridoperator), as long as both prerequisites (minimum electrical power valueP_(emin)and time limit for reaching normal rotor speed) are compliedwith. The above-mentioned rotor and wind speed, moment and torquemeasurements and mechanical rotor power calculations can be used toensure that these constraints are met.

In some further configuration options, the controller is arranged tocontrol rotor speed and/or the electrical output power according to apredetermined (mathematical) function or a control algorithm suitablefor grid stability. Preferably, both parameters are increased accordingto a predetermined gradient. The gradient does not need to be constantor uniform during the overall recovery period. It is, for example,possible that during a first section of the recovery period, theelectrical output power (P_(e)) is not increased at all (but, e.g. isequal to the predetermined minimum value) and only the accelerationpower (P_(acc)) is increased. The additional power which is gained as aresult of increasing efficiency while increasing the rotor speed is, inthis example, completely invested into the rotor acceleration so that(assuming constant wind speed) the rotor acceleration increases furtherand further. Only in a second section of the recovery period, P_(e) isincreased so that the rotor acceleration may not further increase (andcould even decrease again). In this way, the recovery period could bekept relatively short. It is also possible that both parameters arecontrolled and increased in a non-linear manner, for example dependingon the wind turbine's efficiency curve. Further, the controller may bearranged to control the electrical output power to provide a softtransition between overproduction period and recovery period. That meansthat at the end of the overproduction period, the electrical outputpower is not reduced abruptly, but the reduction takes a certain amountof time. By such a “smooth” transition, negative impacts on theelectrical grid and/or the wind turbine can be avoided.

Generally, the recovery period will be of longer duration than theoverproduction period. In the overproduction period, the electricaloutput power is increased, for example in response to informationcontained in the indicating event such as the magnitude of a frequencydrop, the frequency gradient or terminal voltage angle, or to furthercontrol signals received by the controller during the ongoingoverproduction period. The longest possible duration of theoverproduction period generally depends on the kinetic energy stored inthe rotor and the minimum rotational rotor speed. A feasible durationcould, for example, be 10 seconds. In order to continue to contribute tothe grid stability during the recovery period, it is generally notpracticable to re-accelerate the rotor to its normal speed within thesame (relatively short) time frame. On the other hand, the recoveryperiod should not be too extended either, as the wind turbine operateswith a suboptimal efficiency during this time frame. Thus, in somefurther configuration options, the recovery period is preferably fivetimes longer than the overproduction period. In other variants, thefactor is only three, and in still other configurations, the recoveryperiod is twice as long as the overproduction period.

In some optional further configurations, the wind turbine comprises apitch-control system. The controller is then arranged to adjust thepitch angle of the rotor blades during the overall ancillarygrid-stability support mode to mitigate the efficiency reduction of theelectrical output power generation which is caused by the deviation fromthe normal operating point. In particular, the rotor blades can beadjusted during the overproduction period so that the suboptimal angleof wind attack (which is caused by the rotor deceleration) iscompensated. As an effect, the rotor deceleration might be reduced tosome extent. Accordingly, during the recovery period, the pitch anglescan be adjusted while the rotor re-accelerates so that, again, the angleof wind attack is adjusted to the increasing rotor speed. Of course, thepitch-control system can also respond to other changing factors notrelated to the specific grid-stability support mode such as changingwind speeds and/or directions.

Unlike to the operation in partial-load mode, in which the wind turbinegenerally operates at an optimised working point, in full-load mode(i.e. when the available wind power P_(w) correlates to an electricaloperating power P_(e) above the wind turbine's nominal operating powerP_(nom)) wind turbine control limits the turbine's operating power. Thisis accomplished, for example, by turning the rotor blades into the wind,i.e. into the direction of the flag position so that the blades are atleast partially in a flag position (The “flag position” is the bladeposition in which the angle of wind attack is zero. A “partial flagposition” means that the blades are in a position between the optimalangle of wind attack and the flag position so that the angle of windattack is smaller than normal. The term “flagging” designates the degreeto which the blades are turned into the direction of the flagposition.). That means that the angle of wind attack and, as aconsequence, the lifting forces are reduced (in comparison with bladepositioning in partial-load mode) which results in the desiredelectrical output power limitation. In some further configurations ofthe invention, when the wind turbine is operating in a full-load mode,the pitch system is used during the overproduction period to reduce theamount of flagging of the blades (i.e. to re-increase the angle of windattack). By this additional measure, the lift forces at the blades areincreased so that the additional available wind power output is utilisedfor the ancillary control mode. This additional support can, forexample, be used to increase overproduction power (ΔP_(op)) and/or toslow down rotor deceleration and thus to extend the overproductionperiod. Accordingly, during the recovery period, it can be applied toincrease electrical output power and/or to increase rotorre-acceleration, i.e. to increase the acceleration power (P_(acc)), toshorten the recovery period.

For the connection of the variable speed wind turbine to an electricalgrid, the frequency of the wind turbine's electrical sub-system and theelectrical grid are decoupled. In general, there are two ways ofachieving this. Firstly, the wind turbine's generator can be completelydecoupled from the electrical grid by the usage of a full-scalefrequency converter. The generator then produces AC of variablefrequency, which is rectified by a rectifier, and the resulting DC isthen converted to AC with the generally constant grid frequency (50 Hzin Europe). The second alternative is a Doubly-Fed Induction Generator(DFIG). Here, the (asynchronous) generator's stator winding is directlyconnected to the network, i.e. a converter is not used for thisconnection. The exciting field produced by the generator's rotor rotatesrelative to the generator's rotor with a variable speed. The (variable)rotor speed is compensated by correspondingly adjusting the speed of theexciting field relative to the rotor. As a result, the sum of the twospeeds, i.e. the speed of the exciting field relative to the stator isalways a constant value adapted to the fixed grid frequency. A converteris here only needed to produce the exciting-field currents (=the rotorcurrents) with variable frequencies.

Accordingly, in some further configurations of the invention, the windturbine's electrical generator is a generator having a full-scaleconverter. In this solution, a synchronous generator is often used. Dueto the complete decoupling by the full-scale converter, the windturbine's minimum rotor speed (ω_(min)) at which the controllerinitiates the recovery period is not limited by the wind turbine's gridconnection. Rather, the limitation is (only) set by the minimumelectrical output power which the wind turbine (at least) maintainsduring the recovery period and the additional mechanical power (P_(acc))which is needed for rotor re-acceleration.

In other configurations of the wind turbine, a DFIG is used. Here, theoperation range of the converter may be limited by the generator'srotational speed so that the rotational speed of the generator's rotor(and thus the rotational speed of the wind turbine's rotor) may not bedecreased indefinitely while the wind turbine is supposed to generate acertain amount of electrical power. Thus, in these configurations, thisconstruction-dependent lower speed range threshold of the DFIG may applyas an additional threshold for the minimum rotor speed value (ω_(min))at which the recovery period is initiated (in addition to the criterionof the mechanical power (P_(m)) which is needed to provide both theminimum electrical output power which the wind turbine maintains duringthe recovery period and the desired rotor re-acceleration). Thecriterion at which the wind turbine operation arrives first during theoverproduction period (i.e. the higher of the two values) defines thelatest end of the additional electrical power output.

The above considerations were generally based on the assumption ofconstant wind power during the overproduction and recovery periods.Changes in the wind speed will have additional impacts on wind turbineoperation during the ancillary grid-stability support mode according tothe invention. For example, a decrease in wind speed during theoverproduction phase may cause an earlier arrival at the predeterminedminimum rotor speed so that actually less electrical work will be outputto the grid (in comparison with constant wind speed), whereas a windspeed increase may have the opposite effect. The value of the minimumrotor speed (ω_(min)) could also be determined in dependence on the windspeed, e.g. dependent on the average wind speed over the overproductionperiod or another given time window. Furthermore, a decrease in the windpower during the recovery period may lead to a delay in rotorre-acceleration, whereas an increase will enable the wind turbine tore-accelerate the rotor faster in comparison to standard recovery periodoperation. Wind speed variations may also have impacts on a desiredmodulation of the electrical output during the overproduction and therecovery period.

The present invention enables wind turbines to contribute moreefficiently to electrical grid stability. In particular, as windturbines can react relatively fast to grid frequency drops (compared,for example, to primary power control by hydro, gas or steam plants) itcan be used to bridge the gap until the slower power generation plantsstep in.

Finally, the invention allows grid operators to calculate or estimatethe grid-stability support that can be expected from wind turbines fordifferent conditions of the grid and wind speeds.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 illustrates the assembly of a variable speed wind turbine 1schematically. It comprises a rotor 2 which powers an electricalgenerator 9 via a variable speed gear (gearbox) 7. Generator 9 produceselectrical power which is supplied to an electrical grid 17 (not shownin FIG. 1) via electrical line 16. A controller 10 is responsible forcontrolling the wind turbine's 1 sub-systems depending on theenvironmental conditions like wind speed and direction and electricalpower demand.

Rotor 2 comprises a rotor hub 4 and blades 3. In some embodiments, thewind turbine 1 features a pitch control system 6 with which blades 3 canbe pivoted around their longitudinal axis. Thus, it is possible, forexample, to decrease the wind force having an effect on the blades 3(torque) by turning them to the wind. The rotational speed of the rotorω is adjusted depending on the prevailing wind speed (i.e. the windturbine is a variable speed wind turbine, commonly abbreviated: VSWT).Rotor speeds are, for example, in the range between 10 and 20revolutions per minute. Connected to the rotor 2 is a gearbox 7 whichserves to convert the relatively slow rotational rotor speed ω into ahigher rotational speed of the generator's 9 rotor (in otherembodiments, the wind turbine 1 does not have a gearbox, but of courseis also be arranged to perform grid-stability support activity). Break 8allows the rotor speed to be decreased, for example, in order to shutdown the wind turbine 1. The internal mechanical, electrical and controlsub-systems are housed in a nacelle 5 which is mounted on tower 15.

A further component of the wind turbine 1 is, in some embodiments,sensor and measurement equipment 11. For example, an anemometer 12serves to determine the current wind speed, while a wind vane 13provides wind direction measurements. Further, a measurement device 36may be provided for measuring the moment at the root of the rotosblade's 3 and measurement device 37 for measuring rotos shaft rotationalspeed and/or torque. Finally, in some embodiments, yaw drive 14 allowsthe nacelle 5 with rotor 2 to be adjusted around the vertical (tower)axis according to the prevailing wind direction.

Controller 10 is, for example, arranged as a microprocessor withassociated memory which executes control software. In some embodiments,it is arranged as a single component, whereas in other embodiments it ismade up of distributed sub-systems, for example, in the form of severalmicroprocessors connected to each other. It is coupled via a bus to thewind turbine's sub-systems, in particular to generator 9, rotor 2,pitch-control system 6, sensor equipment 11, i.e. anemometer 12, windvane 13 and the measurement devices 36 and 37, and yaw drive 14.Furthermore, it is connected to management system 18 (not shown in FIG.1).

FIG. 2 exemplarily shows the control flow of the wind turbine 1 in moredetail. Controller 10 receives measurement results provided by sensorequipment 11 (anemometer 12, wind vane 13 and the measurement devices 36and 37) and operating parameters from the electrical grid 17 andgenerator 9. It processes these input data and generates control signalswhich are transmitted to actuators of the various sub-systems of thewind turbine 1. The latter execute the control commands received fromcontroller 10 and effect a change of the respective sub-system's state.For example, controller 10 manipulates the rotor speed w by changing thegenerator frequency or by varying the gear ratio (control flow of lattermanipulation is not shown in FIG. 2).

Furthermore, controller 10 also receives commands from management system18 which may be located within or outside the wind turbine 1. A remoteoperation monitoring equipment 19 such as a Supervisory Control and DataAcquisition (SCADA) system is provided to supervise the wind turbine's 1operation. For this purpose, it requests data about the wind turbine's 1state from management system 18 and conducts remote parameterisation(i.e. commands regarding parameter settings are transmitted tomanagement system 18).

Two tasks performed by controller 10 are of special interest with regardto the various embodiments: Firstly, it is arranged to monitor eventsindicating a need to increase the electrical output power beyond thenormal operating power. Such an indicating event can, for example, bereceived by remote operation monitoring 19 (via management system 18)or, alternatively, be generated by controller 10 itself, both inresponse to measurements of electrical grid parameters such as the gridfrequency (for example, provided by a grid frequency analyser, not shownin FIG. 2) or the voltage angle change. Secondly, controller 10 isarranged to initiate the ancillary two-phased control mode according tothe present invention.

The wind turbine 1 generates electrical power when the wind speed iswithin the turbine's operating range. Its (static) production curvewhich is exemplarily illustrated in FIG. 3 (as thick black line) as afunction of rotational rotor speed w and electrical output power P_(e)(and considering different wind speeds) is non-linear. Below the lowerrotor speed threshold (ω_(thresh), no electrical power is produced.Starting at the lower rotor speed threshold ω_(thresh), P_(e) at firstincreases vertically for low wind speeds. With increasing wind speeds,the rotor speed ω is increased according to the efficiency curves forthe different wind speeds until the nominal rotor speed ω_(nom) isreached. Although rotor 2 is not accelerated further, P_(e) can still beboosted (again with further increasing wind speed), until the windturbine's nominal electrical output power has been reached (P_(nom)=1.0pu (i.e. power unit) in FIG. 3). As can be seen in FIG. 3, theproduction characteristic line deviates from the theoretically possibleoptimal curve (the thin black line) to some extent. However, as thistheoretical curve is not feasible in practice, wind turbine operation onthe (thick) production line is considered to be optimal. Depending onthe wind turbine's 1 generator type, its operation range is howeverdynamic to a certain extent (which is marked by the striped area in FIG.3), i.e. the actual power output P_(e) and rotational rotor speed ω candeviate to a certain extent from the (optimal) static production line.

Three embodiments which illustrate different wind turbine operationsduring the recovery phase will now be described. According to the firstembodiment (FIGS. 4 and 5), the rotor is re-accelerated by using a fixedamount of the available mechanical rotor power P_(m) which effects anincrease of the electrical output power P_(e) from the beginning of therecovery period. The second embodiment (FIGS. 6 and 7) follows adifferent approach. Here, P_(e) is at first kept at the minimum levelwhich is required during the recovery period (P_(emin)). As aconsequence, the power available for rotor re-acceleration increaseswith the increasing efficiency resulting in a shorter recovery periodcompared to the first embodiment. The third embodiment (FIG. 8, incombination with FIG. 4) is a specific modification of the firstembodiment. In the first phase of the recovery period, all availablemechanical rotor power P_(m) is used for the generation of P_(e) sothat, in this phase, the rotor is not accelerated at all. Only in thesecond phase, P_(e) is slightly decreased and the rotor isre-accelerated by using the now available part of P_(m) in accordancewith the scheme presented in the first embodiment. For all embodiments,constant wind speed during the overproduction and recovery periods isassumed.

The values of P_(e), P_(m) and ω during the overproduction and recoveryperiods in the first embodiment are visualised in FIG. 4 (P_(e) is thesolid line, P_(e0) the dash-dot line, P_(m) the dotted curve andP_(emin) the dash-dot-dot line). During normal operation, the windturbine operates on the working point with an optimal efficiency, i.e.the operating power P_(e) and rotor speed ω are set in accordance withits production curve (cf. FIG. 3). When controller 10 detects anindicating event it initiates the ancillary grid-stability supportactivity according to the invention. At the moment the wind turbine 1enters the first phase, i.e. the overproduction period, electricaloutput power and rotor speed correspond to point A in FIG. 4 (P_(e0) andω₀ wherein P_(e0)=0.8 pu in the example of the first embodiment). Withthe start of the overproduction period, P_(e) is increased beyond itsnormal value, shifting the wind turbine's working point to point B inFIG. 4. This additional power is called ΔP_(op). In the embodiments,ΔP_(op) is assumed to be constant during the duration of theoverproduction period (as outlined above, it could, however, also bevaried over time). As ΔP_(op) is taken from the kinetic energy stored inrotor 2, ω decreases with ongoing output of the increased electricaloutput power, so that the wind turbine's working point changes frompoint B to C in FIG. 4. As also indicated in FIG. 4, the mechanicalrotor power P_(m) decreases due to the worsening efficiency thataccompanies the decreasing rotor speed (cf. efficiency curves in FIG.3).

Controller 10 continuously measures ω and monitors generator's 9operation. It stops the increased electrical power output (at thelatest) at the moment the respective minimum value ω_(min) is reached(point C in FIG. 4). The arrow from point C to point D marks the end ofthe overproduction and the beginning of the recovery period. With therotational rotor speed being ω_(min) the available mechanical rotorpower P_(m) is approximately 0.7 pu (as opposed to 0.8 pu in normal windturbine operation in point A). This reduced amount of available P_(m) isnow the base for re-accelerating rotor 2 while further outputtingelectrical power to the electrical grid 17 and thus graduallyre-shifting the wind turbine's 1 operating point from point D back topoint A. In the first embodiment, a constant power amount (P_(acc)) isused for rotor re-acceleration, namely ca. 0.025 pu. The remaining partof the available P_(m) is used for electrical power production (itapplies: P_(m)−P_(e)P_(acc), neglecting efficiency curtailment). Withincreasing rotor speed, the efficiency improves again, and, accordingly,the electrical output power increases (while rotor 2 is stillaccelerated using the constant P_(acc)). During the recovery period,P_(e) never drops below the predetermined minimum electrical outputpower which is exemplarily set to 0.65 pu in the first embodiment (i.e.81.25% of P_(e0)). The recovery period ends when the rotor speed arrivesagain at the nominal speed ω₀, and the wind turbine 1 continues tooperate at its normal operating point (point A in FIG. 4).

The first embodiment of the present invention can also be visualised byFIG. 5 (overproduction period: points A through D, recovery period: D toA′). The upper diagram shows P_(e) (solid line) and P_(m) (dotted line)over time, whereas the lower diagram depicts the rotational rotor speedω. As only a relatively small amount of power is invested in rotorre-acceleration, the recovery period is about four times longer than theoverproduction period. Due to the constant P_(acc), the rotational rotorspeed is increased constantly during the recovery period.

A variation of the first embodiment is shown in FIG. 5 a, wherein theelectrical output power is modulated during both, overproduction andrecovery period. Such modulation is, for example, performed in responseto ongoing grid parameter measurements which results are transmitted tovia management system 18 to controller 10, based on stipulations by thegrid operator (which are, e.g., included in the indicating event) orwind speed variations. By performing such output power modulation, animproved contribution of wind turbine 1 to the stability of theelectrical grid is possible. The rotational rotor speed ω variesaccordingly. Furthermore, the transition from overproduction to recoveryperiod does not need to be an abrupt reduction of the electrical outputpower (as shown in FIG. 5), but may be soft, e.g. according to amathematical function (the transition could be significantly “smoother”than indicated in FIG. 5 a).

The second embodiment (FIGS. 6 and 7) is mainly characterised by adifferent utilisation of P_(m) during the recovery period. Again, thetwo-phased ancillary grid-stability supporting mode is initiated bycontroller 10 after it has detected an indicating event. The electricaloutput power P_(e) is raised to 0.9 pu (P_(e0) being 0.8 pu as also inthe first embodiment) so that the wind turbine's 1 working point changesfrom point A to point B and—while the rotational rotor speeddecreases—further on to point C. In the second embodiment, the minimumrotational rotor speed ω_(min) is slightly lower than in the firstembodiment (for example, because the wind turbine according to thesecond embodiment has a full-scale converter instead of a DFIG, whichallows for a larger deviation of generator speed and electrical gridfrequency). Accordingly, the overproduction period is slightly longer(with same ΔP_(op) as in the first embodiment). When ω_(min) is reached,the controller again initiates the recovery phase. In the secondembodiment, the minimum electrical output power P_(emin) during therecovery period is set to 0.64 pu (which equates to 80% of P_(e0)=0.8pu). The available mechanical rotor power P_(m) at point D is onlyslightly above this lower threshold P_(emin.) Controller 10 now controlsthe wind turbine 1 in a way that for the first part of the recoveryperiod only the minimum electrical output power is actually produced(i.e. P_(e)=P_(emin) during this time frame). In this way, the powerused for re-acceleration of rotor 2 increases with the increasingefficiency (or in other words: the “gap” between P_(m) and P_(e)increases over this first part of the recovery period as P_(m) increaseswith higher ω and P_(e) stays constant). Only in a second part of therecovery period—when ω has already nearly reached ω₀−P_(e) is(relatively fast) increased and, accordingly, P_(acc) is decreased.

The effect of this procedure is a shorter recovery period because ω₀—incomparison to the first embodiment—is reached significantly faster (FIG.7). Hence, the recovery period is here only about twice as long as theoverproduction period. However, this advantage is achieved only at theexpense of a lower P_(e) during the main part of the recovery period.

The third embodiment (FIG. 8) basically follows the diagram of FIG. 4,presented in connection with the first embodiment. However, in contrastto the first embodiment, the recovery period is now two-phased. In afirst phase (from point D′ to D in FIG. 8), the complete mechanicalrotor power is converted into electrical output power (i.e.P_(m)=P_(e)). As a result, no power is left for rotor re-acceleration(P_(acc)=0 and rotor speed ω stays constant at ω_(min)). The windturbine's working point equals the point of intersection between arrowC-D and the P_(m) curve in FIG. 4. In a second phase (from D to A′ inFIG. 8), the electrical output power is slightly decreased (but still ator above the minimum P_(emin)) so that a portion of P_(m) is releasedfor rotor re-acceleration. The re-acceleration is then performed similarto the first embodiment.

The third embodiment permits a slightly higher contribution to thestability of the electrical grid 17 in the first phase of the recoveryperiod. Of course, the recovery period is extended by the duration ofthe zero rotor re-acceleration so that, in the example of FIG. 8, it isabout 6.5 times longer than the overproduction period.

The embodiments one to three have been described so far on theassumption that the wind turbine 1 operates in partial-load mode when anindicating event is detected by controller 1 and the ancillary controlmode is initiated. Of course, the wind turbine 1 can also performgrid-stability supporting activity while operating in full-load mode(i.e. P_(e0)=P_(nom), with available wind power P_(w) corresponding to ahigher power than the nominal electrical power of the wind turbine, cf.FIG. 9). In this case, the overproduction period, in general, does notdiffer from partial-load operation (apart from the fact that, as thewind turbine 1 will be operating at its nominal operating power prior tothe overproduction period, the output power increase during theoverproduction period will cause a temporary power output whichtemporarily exceeds the nominal power, but still lies within the windturbine's tolerance range). The increased electrical output power ismaintained until the minimum rotation rotor speed ω_(min) is reached.The overproduction period may be longer than in partial-load mode due tothe increased kinetic energy stored in rotor 2 and additional support byblade pitching, depending on the respective P_(emin) and the operatingconstraints of generator 9, i.e. the wind turbine's 1 dynamic operationrange (cf. FIG. 3). In the embodiment according to FIG. 9, the bladesare deflagged during the overproduction period, thus gaining additionallift force that can be converted into additional electrical power orused to reduce rotor deceleration. Similarly, the recovery period can beshortened by utilising the excess wind power for rotor re-accelerationin a similar manner. In the example of FIG. 9, the overproduction periodis therefore longer (in comparison with the previous embodimentsreferring to partial-load mode, cf. e.g. FIG. 5), and the recoveryperiod is only twice as long as the overproduction period.

In all embodiments described, the wind turbine 1 may, for example, beequipped with a synchronous generator having a full-scale converter(FIG. 10) or, alternatively, with a Doubly-Fed Induction Generator(DFIG) (FIG. 11). In the first case, 100% of the generated electricalpower P_(e) passes through the converter 20 which has a rectifier 21 andan inverted rectifier 22. In the latter case, stator winding 23 isdirectly connected to the electrical grid 17 and the rotor-gridconnection is realised by using a converter 20 which can, for example,be a cascading converter with DC link (cf. FIG. 11). Hence, only about30% of the produced electrical output power P_(e) passes throughconverter 20 while the main portion is fed directly into the electricalgrid 17.

The duration of the recovery period is not only dependent on the powerused for rotor re-acceleration (P_(acc)), but also on the prevailingwind speed. FIG. 12 visualises durations of the recovery period forcontrol modes according to the first embodiment (fixed amount ofre-acceleration power P_(acc) during the recovery period) in dependenceon the wind speed. In a first variant, P_(acc) is chosen to berelatively small (0.02 pu) so that the recovery period is relativelylong. If P_(acc) is set to higher amounts (0.06 pu and 0.1 pu in FIG.12), the duration of the recovery period generally decreases. Higherwind speed generally tends to result in a longer duration. Inparticular, if P_(acc) is set to a relatively small value (0.02 pu inFIG. 12), the duration of the recovery period increases significantlywith higher wind speeds. With wind speeds above the nominal value(full-load operation), the recovery period, however, decreasesdrastically (if, different from FIG. 9, the same amount of P_(m) is usedfor rotor acceleration as in partial-load mode).

According to one aspect of the present invention, a wind turbine controlmethod for grid-stability support is provided (FIG. 13). Generally, thewind turbine 1 operates according to its standard policy 30 (e.g.maximum power tracking, cf. also the production curve in FIG. 3). Eventswhich indicate a need for additional P_(e) supply are monitored during30. The monitoring may also include measurement of grid parameters (suchas frequency variations or voltage angle change), the determination ofthe extent of grid support and (detailed) instructions to wind turbine1. When such an indicating event is detected (arrow 31), the windturbine 1 executes the overproduction period at 32. As described indetail above, kinetic energy is extracted from rotor 2 and convertedinto additional electrical power ΔP_(op) while rotor speed ω decreasesmore and more. The electrical output power may be varied or modulated.The recovery period may be initiated prior to reaching the minimum rotorspeed ω_(min), for example, when the electrical grid appears to bestable again, an elapsed time or wind turbine internal operatingconditions such as overheating of components or wind turbine load.However, it is entered, at the latest, when the minimum rotor speedω_(min) is reached (arrow 33). In the recovery period, the rotor 2 isre-accelerated again and at least a certain amount of P_(e) is stillsupplied to the electrical grid 17. Other activities dependent on theprevious grid-stability support may also be performed during therecovery period in order to restore normal wind turbine operation, suchas the cooling down of heated components, the dampening of mechanicalosciallation etc. When the rotor 2 arrives at its normal rotationalspeed (e.g. ω₀, assuming unchanged wind speed since entering theoverproduction period at 32), the wind turbine 1 returns to its normaloperation (arrow 35).

The wind turbine functionality and control method described and claimedherein is not exclusive. The wind turbine described herein may bearranged to perform further grid-stabilising activities besides thedescribed ones, such as decreasing power generation in a transientmanner in the case of a sudden grid power oversupply (e.g. when asignificant power consumer is disconnected from the grid) orcounteracting periodic disturbances (e.g. grid frequency oscillations)etc.

1. (canceled)
 2. A wind turbine for connection to an electrical grid,the wind turbine comprising: a rotor with blades coupled to anelectrical generator; and a controller configured to: increase, inresponse to an event, an electrical output power from the wind turbineusing kinetic energy stored in the rotor thereby decreasing a rotationalspeed of the rotor, wherein the event indicates a need to increase theelectrical output power from the wind turbine to the electrical grid tosupport the stability of the electrical grid, and after increasing theelectrical output power and before the rotational speed of the rotorreaches a predetermined minimum value, accelerate the rotor to aprevious rotational speed while outputting at least a predeterminedminimum electrical power to the electrical grid.
 3. The wind turbineaccording to claim 1, wherein the controller is configured to controlthe wind turbine such that the predetermined minimum output poweroutputted during an recovery period is a fixed percentage of anelectrical operating power outputted by the wind turbine before theevent occurs.
 4. The wind turbine according to claim 1, wherein thecontroller is configured to control the wind turbine such that thepredetermined minimum electrical power is at least 80% of an electricaloperating power which was supplied to the electrical grid prior to theevent occurring.
 5. The wind turbine according to claim 1, wherein thecontroller controls the power used to accelerate the rotor such that apredetermined minimum rotor acceleration is ensured.
 6. The wind turbineaccording to claim 1, further comprising equipment for measuring atleast one of: the rotational speed of the rotor, a wind speed, a momentat a root of one of the blades and the torque at a rotor shaft, andwherein the controller is configured to control a power used toaccelerate the rotor by continuously measuring at least one of thefollowing parameters: the rotational speed of the rotor, the wind speed,the moment at the root of one of the blades and the torque at the rotorshaft and calculating the available mechanical rotor power from at leastone of the parameters.
 7. The wind turbine according to claim 1, whereinthe controller is configured to accelerate the rotor when at least oneof: the electrical grid is stabilized, a duration of an overproductionperiod where the electrical output power is increased reaches a timelimit, and an amount of additional energy provided to the electricalgrid during the overproduction period reaches a predefined limit.
 8. Thewind turbine according to claim 1, wherein the controller is configuredto control at least one of the rotational speed of the rotor and theelectrical output power provided by the wind turbine when the rotor isbeing accelerated such that the previous rotational rotor speed isreached within a predetermined time period.
 9. The wind turbineaccording claim 1, wherein the controller is configured to control atleast one of the rotational speed of the rotor and the electrical outputpower provided by the wind turbine when the rotor is being acceleratedsuch that at least one of the rotational speed and electrical outputpower is varied during the recovery period according to a predeterminedfunction with a predetermined gradient.
 10. The wind turbine accordingto claim 1, wherein the controller is configured to control the windturbine such that a duration of a recovery period when the rotor isbeing accelerated is at least twice as long as a duration of anoverproduction period when the electrical output power is increased. 11.The wind turbine according to claim 1, further comprising a bladepitch-control system, wherein the controller is configured to adjust ablade pitch angle when at least one of a overproduction period when theelectrical output power is increased and a recovery period when therotor is being accelerated based on a rotor speed change so thatefficiency decrease of the power conversion is reduced during at leastone of the overproduction and recovery periods.
 12. The wind turbineaccording to claim 10, wherein the wind turbine operates in a full-loadmode with the blades in an at least partial flag position, wherein thecontroller controls the wind turbine such that, if available wind powercorresponds to a higher electrical power than a nominal operating powerof the wind turbine, the amount of flagging is reduced during at leastone of the overproduction and recovery periods so that at least aportion of the available wind power is used to at least one of: reducede-acceleration of the rotor, increase re-acceleration of the rotor, andincrease the electrical output power.
 13. The wind turbine according toclaim 1, wherein the controller is configured to identify the eventbased one at least one of a grid frequency change and a voltage anglechange.
 14. The wind turbine according to claim 1, wherein theelectrical generator is a full-scale generator, wherein thepredetermined minimum value is a minimum speed at which the wind turbineis operable to produce a minimum power output required to accelerate therotor to the previous rotational speed.
 15. The wind turbine accordingto claim 1, wherein the electrical generator is a doubly-fed inductiongenerator wherein the predetermined minimum value is the highest valueof one of the following: a minimum speed at which the doubly-fedinduction generator is operable to produce a minimum power outputrequired to accelerate the rotor to the previous rotational speed, and aconstructional lower threshold of a rotational speed range of thegenerator.
 16. A controller controlling the operation of a wind turbinehaving a rotor with blades and being arranged for connection to anelectrical grid, the controller is configured to perform stepscomprising: increase, in response to an event, an electrical outputpower from the wind turbine using kinetic energy stored in the rotorthereby decreasing a rotational speed of the rotor, wherein the eventindicates a need to increase the electrical output power from the windturbine to the electrical grid to support the stability of theelectrical grid, and after increasing the electrical output power andbefore the rotational speed of the rotor reaches a predetermined minimumvalue, accelerate the rotor to a previous rotational speed whileoutputting at least a predetermined minimum electrical power to theelectrical grid.
 17. A method of controlling a wind turbine, the windturbine having a rotor with blades and being arranged for connection toan electrical grid, the method comprising: increasing, in response to anevent, an electrical output power from the wind turbine using kineticenergy stored in the rotor thereby decreasing a rotational speed of therotor, wherein the event indicates a need to increase the electricaloutput power from the wind turbine to the electrical grid to support thestability of the electrical grid; and after increasing the electricaloutput power and before the rotational speed of the rotor reaches apredetermined minimum value, accelerating the rotational speed of therotor to a previous rotational speed while outputting at least apredetermined minimum electrical power to the electrical grid.